Magnetrons



Aug. V30, 1955 F. c. THOMPSON ET AL 2,716,711

MAGNETRONS Filed Dec. 27, 1951 2 Sheets-Sheet l 1955 F. c. THOMPSON ETAL 2,716,711

MAGNETRONS Filed Dec. 27, 1951 2 Sheets-Sheet 2 United States Patent iMAGNETRONS Application December 27, 1951, Serial No. 263,643

Claims priority, application Great Britain January 11, 1951 4 Claims.01. 313-30 This invention relates to magnetrons and more specifically tomagnetrons of the so-called packaged type i. e. the type in which, inorder to reduce the length of the air gap in the magnetic circuit, thedischarge envelope is completed by members of magnetic material whichalso form part of the magnetic circuit so that, in effect, the magneticpoles are brought inside the envelope.

Although packaged magnetrons have the advantage over the non-packagedtypes that, owing to the improved magnetic system they can be made ofsubstantially reduced weight and size, this advantage is accompanied byimportant difl iculties and disadvantages the nature of which will bebetter understood upon reference to the accompanying drawings in whichFig. 1 shows, in section,

the essential parts of a typical known magnetron of the non-packagedtype; Fig. 2 is a similar view of a typical known packaged magnetron;and Fig. 3 is a simplified sectional view showing the general pole pieceand cathode structure arrangement of a magnetron embodying the presentinvention.

In Fig. 1 the cathode 1, heated by an internal heater 2 is axiallysituated within a copper anode structure 3 having anode segments 4. Thestructure 3 forms part of the discharge space envelope which iscompleted by flat copper side plates 5 and located between the poles 6of the magnet system. The magnetic gap is, as will be seen, a long one,being indicated by the dimension M. The cathode structure is supportedby support members 7 which enter the envelope from the side and extendmore or less at right angles to the axis. End caps in the form of discs8 prevent electrons leaving the anode cathode space and being collectedby parts of the anode structure other than the anode segments. The discs8 thus serve to maintain a high conversion efficiency but their presenceas well as that of the transverse support structure for the cathode, andthat of the side plates 5, results in a long air gap M with consequentnecessity to provide a heavy and bulky magnet system.

In the typical known packaged magnetron shown in Fig. 2 the air gaplength M is much reduced with consequent ability to use a much strongermagnetic field with the same or a smaller magnet system by dispensingwith the side plates 5, using the pole pieces 6 as part of the envelopestructure, and supporting the cathode structure by means, generallydesignated 7a which run axially through a hole in one of the pole piecesinstead of transversely as in Fig. 1. However, it is a requirement thatthe magnetic field in the space between anode and cathode shall be asuniform as possible and run as nearly as possible parallel to thecathode surface and this requirement strictly limits the maximumdiameter which can be tolerated for the hole in the pole piece. For amagnetron to work efficiently the lines of magnetic force in theanodecathode space should run as nearly as possible parallel to the axisof the anode and cathode and any departure from this ideal parallelismshould be such as to make the field barrel-shaped and not in the otherdirection i. e.

2,71 6,7 l l Patented Aug. 30, 1955 such as to give the field a waistbetween the opposed pole faces. Any waisting of the field will tend tocause electrons near the ends of the cathode to spiral outwards and awayfrom the anode with consequent losses of power and lowered conversionefficiency. Any hole in the pole piece tends to produce some waisting ofthe field since the lines of force tend to spread into the hole. Inknown packaged magnetrons therefore the said hole has had to be kept ofvery small size and this in turn means in practice that either thecathode support tube in the hole must be made very thin and consequentlyweak mechanically or the spacing between the support tube and thesurrounding pole piece must be kept smaller than is desirable. In thisconnection it will be remembered that, in a known packaged magnetron,the pole piece is at anode potential and in consequence the fullanode-cathode potential is applied between said pole piece and thesupport tube. Since there is very little magnetic field inside the polepiece hole any electron emission from the cathode support tube will becollected by the pole piece acting as the anode of a simple diode. Inknown packaged magnetrons there is, therefore, the probability of theexistence of a diode in parallel with the magnetron proper and thisdiode is capable of absorbing a large amount of power, with consequentfurther loss of efficiency. This diode effect can grow seriously duringthe life of a magnetron for barium or other emissive material canmigrate from the cathode to the cathode support tube and since the saidcathode is usually mounted directly on the said tube the operatingtemperature of the latter is usually high enough for it to emit readily.

In addition the necessity for strictly limiting the diameter of the holein the pole piece involves that it is impracticable in known magnetronsto fit large end caps such as the caps 8 of Fig. 1, for a cathodestructure with such caps would not pass, in assembly, small hole in thepole piece and, in general, it is, if not absolutely impossible, at anyrate very diflicult and impracticable for manufacturing reasons to fitthe end caps to the cathode structure after the latter has been mountedon the pole piece.

It has therefore been the practice, with known packaged magnetrons,either to omit end caps altogether or to be content with very small endcaps with resultant waste power and loss of efficiency.

The present invention seeks to avoid or eliminate the above difficultiesand defects.

According to this invention a packaged magnetron wherein the cathodestructure includes a cathode support member mounted within a hole in apole piece is characterized in that said support member is mounted in asheath of magnetic material at cathode potential and arranged to havegood thermal insulation with respect to the cathode.

In the preferred embodiment of the invention the cathode support memberis a tube inserted into a sheath of magnetic material which is mountedwithin a hole in the pole piece and insulated therefrom, said sheathbeing thermally insulated from the cathode by a long conducting path.

Fig. 3 illustrates an embodiment of the invention. Here the pole pieces6a, 6b are alike and provided with axial holes which may be made aslarge in diameter as may be required. The cathode which is indirectlyheated by an internal heater, not shown, is on a cathode support tube 7.The end of the support tube 7 is fitted into the end of a sheath 8 ofmagnetic material which is mounted in the pole piece 6a and is atcathode potential, being insulated from the said pole piece. The sheath8 is preferably shaped as shown being undercut at 81. By virtue of thisundercutting and of the general arrangement illustrated the surface ofthe sheath opposite the through the necessarily wall of the hole in thepole piece will be much cooler than the cathode and cathode supporttube, and it will be obvious that it is unlikely to be appreciablycontaminated by electron emitting material from the cathode. Themagnetic field can be made to approach closely to the ideal since thediameter of the hole in the magnetic material including pole piece andsheath within it may be made equal to or even less than that of thecathode despite that the pole itself has a large hole. Further thearrangement enables a mechanically strong cathode structure to beemployed with little liability to becoming eccentric in use.

In the illustrated embodiment the lower pole piece 6b is fitted with aninternal magnetic sheath 8a to ensure symmetry of the magnetic fieldabout the transverse midplane of the anode, not shown.

Of course the material of which the sheath 8 is made must be such as toensure that it remains magnetic at its operating temperature. Suchmaterials are commercially available.

We claim:

1. A magnetron structure comprising an axial cathode structure having acathode on a cathode support tube, an anode structure concentricallysurrounding said cathode, a pair of magnetic pole pieces, one adjacenteach end of said cathode and adapted to provide magnetic lines of forcethreading said cathode in a direction substantially parallel to the axisof said cathode, an evacuated envelope of which part is constituted bysaid anode structure and parts by said pole pieces, one of said polepieces being tubular, and a sheath of magnetic material mounted within,and electrically insulated from, said one tubular pole piece, saidcathode A support tube having its end inserted into the end of saidsheath adjacent to said cathode supporting said cathode from said sheathand electrically connecting said cathode to said sheath.

2. A structure according to claim 1 wherein said support tube isundercut and forms a constricted linearly extending heat conductingpath.

3. A structure as claimed in claim 1 wherein the other of said polepieces is also tubular and is substantially colinear with the firstmentioned tubular pole piece and a second sheath of magnetic materialfitted in said last mentioned tubular pole piece.

4. A structure according to claim 1 wherein said support tube isundercut and forms a constricted heat conducting path and wherein theother of said pole pieces is also tubular and is substantially co-linearwith the first mentioned tubular pole piece and a second sheath ofmagnetic material fitted in said last mentioned tubular pole piece. t

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES A. I. E. E. Technical Paper 5266, Dec. 12, 1951. See Figs. 2and 3 and pages 5 and 6.

